Amperometric sensors are based on the simple principle that reducible or oxidizable substances can be converted by applying a corresponding potential to an electrode. The so-called Faraday current that then flows is a measure of the concentration of this substance. This current cannot, however, be measured directly after the potential has been applied, because above all a high capacitive current flows in the first instance. This is brought about by the charge-reversal of the double layer of the electrode. It dies down exponentially with time.
The Faraday current is likewise reduced with time, since as a result of the substance-conversion, a depletion takes place in front of the electrode. The subsequent delivery of substance from the solution to the electrode takes place solely as a result of diffusion in the case of non-stirred, that is, non-convective systems. Concentration profiles set in the solution. With sufficiently positive potentials (oxidation) or negative potentials (reduction), the Cotrell equation holds good for the time-dependence of the current:
                              I          =                      nFA            ⁢                                                  ⁢                                          D                                                              π                  ⁢                                                                          ⁢                  t                                                      ⁢                          c              ∞                                      ,                  s          ->          ∞                                    (        1        )            
The significations in this connection are:    n: number of electrons per formula conversion    F: Faraday constant    A: area of the electrode    D: diffusion coefficient    c∞: concentration of the substance to be converted in the solution    s: extent of the electrolyte space in front of the electrode
Equation (1) implies that the Faraday current diminishes with the reciprocal square root of time, that is, more slowly than the capacitive current. Some time after the change in the potential, the current is therefore mainly determined by the Faraday current. For each system it is necessary to find a point in time at which the capacitive current is low, yet the Faraday current is still as high as possible.
One possibility for determining very small concentrations (10−10 mol/l) is voltammetric stripping analysis. In this connection, by applying a suitable potential the substance that is to be determined is first enriched by that of the electrode. In a second step, the quantity of substance on the electrode is then determined [W. Buchberger, Elektrochemische Analyseverfahren, Heidelberg Berlin: published by Spektrum Akademischer Verlag in 1998 and DE 44 24 355]. This method does not enable there to be any time resolution on account of the two method steps. Changes in concentration cannot therefore be tracked or can only be tracked very slowly.
A further analytical standard method for determining very small concentrations is polarography [Rach & Seiler, Polarographie u. Voltametrie in der Spurenanalytik, Heidelberg: Hüthig 1984]. Here concentrations in the order of magnitude of 10−9 mol/l can be measured with suitable pulse methods and with the use of a so-called “Dropping Mercury Electrode” (DME). In the case of the DME, use is made inter alia of the fact that each drop of mercury plunges into the solution anew and the concentration c∞ is found there again. In the case of a static electrode, however, this is not the case. After each measurement it would be necessary to wait for so long until the concentration profile is relaxed by the diffusion process. Depending on the magnitude of the diffusion coefficient, if there is no stirring this process can take a few seconds. If the waiting time is not long enough, the current will diminish from measurement to measurement, since the initial concentration drops in front of the electrode.
In the case of some applications, however, it can be necessary to determine the concentration in rapid succession, if, for example, a change in concentration is to be observed. One possible area of use is that of tracking the enzymatic formation of a mediator in molecular-biological detection systems.
In the case of biochemical sensors, molecular recognition systems, for example haptens, antigens or antibodies, are placed on or in the vicinity of the electrodes. The target molecule binds thereto and is provided with an enzyme label either directly or by way of intermediate steps. If the corresponding enzyme substance is now added, the enzyme releases a substance that can be detected. This happens either visually or electrochemically. This is the so-called ELISA-test (Enzyme Linked Immuno Sorbent Assay). DNA analysis methods can also be carried out in a similar way.
In the case of electrochemical detection, it is advantageous to detect not just the absolute concentration of the measured variable, referred to as a “mediator”, for the electrochemical conversions specified above, but also the change, in particular the increase, in the concentration over a few seconds. As a result, the influence of different states of the biochemical system is eliminated at the start of the measurement. The time resolution of such a measurement must then amount to 1 to 2 Hz in order to be useful technically and economically.
Specifically in the case of redox-active substances up until now it has been possible to employ so-called redox-cycling with the use of interdigital electrodes. In so doing, use is made of the fact that the substance that is oxidized at one electrode can be reduced again at the second electrode. The electrodes are then constantly set to the oxidation or reduction potential. To this end, the two interdigital electrodes with electrode fingers interlocking in a chamber-like manner are connected together with the reference electrode and a counter electrode to a bipotentiostat [O. Niwa, M. Morita, H. Tabei, Anal. Chem. 62 (1991), 447-452 and DE 43 18 519 A1].
A precondition for redox-cycling is that the distance between the electrodes, that is, the electrode fingers of the interdigital electrodes that are associated with each other, lies in the order of magnitude of the diffusion-layer thickness, that is, in the region of a few μm. On the basis of the concentration profiles, in addition to the concentration and the diffusion coefficient the number of electrode fingers and their length enter into the measured current [K. Aoki, J. Electroanal. Chem, 270 (1989), 35]. It follows from this that the necessary structures must be very fine and expensive to produce.
The monograph “Electrochemical Methods, Fundamentals and Applications”, John Wiley & Sons, 1980, provides a general overview of electrochemical measuring technology. Further references to measurement specifically with respect to liquids or even for biochemical measurements are given in DE 43 35 241 A1, in DE 41 31 731 A1, in DE 197 17 809 U1 and DE 199 17 052 A1. A method for electrochemically measuring redox-cycling with a practice-related electrode arrangement is described in detail in WO 01/67587 A1.